Field of the Invention.
The present invention generally relates to testing of semiconductor devices, and, more particularly, to a method and test probe card for testing large format semiconductor devices, such as flat panel image sensor devices.
Relevant Background.
Current state of the art methods of testing semiconductor devices require a means of providing temporary electrical contacts to the device under test without damaging the connection surfaces that will later be used in the device packaging after successful testing of device.
Current Semiconductor Industry temporary test contact technologies consist of two basic methods including Probe Cards and Flexible Circuits.
Probe Cards are now discussed. This test contact method utilizes a mechanical assembly that serves as an interface between an electronic test system and the Device Under Test, herein known as “DUT”. Typically the probe card is mechanically docked to a prober and electrically connected to a tester. Its purpose is to provide an electrical path between the test system and the circuits on the wafer, thereby permitting the testing and validation of the circuits at the wafer level, usually before they are diced and packaged. This probe head generally utilizes machined alloy housings combined with a rigid circuit board that will contain many individual mechanical pins that each serves as a contact point for a single device line to be tested. Each probe can contain as few as ten to twenty contact pins, or up to thousands of total pins depending on the specific application. Probe cards are broadly classified into three types:
Needle type: These pins are made from wire with bends to provide upward resisting spring force when moved into contact with the DUT.
Vertical type: These pins are also made from wire but are placed in a vertical orientation with the beam rigidity of the wire serving as the spring loading force for the pin contacts. These types of cards generally have a design where each pin will have its own spring loaded housing, also known as pogo pins.
MEMS (Micro Electro-Mechanical System) type depending on shape and forms of contact elements. MEMS type is the most advanced technology currently available. This most advanced type of probe card currently can test an entire 12″ wafer with one touchdown. The micro-machined nature of this type of contact method makes this solution very expensive.
Because each probe card is manufactured to very exact tolerances for pin placements in all axes, all three types of probe cards are very expensive to build and to maintain and are not scalable to different product sizes without all new tooling. Additionally, manufacturers will typically require a new probe card for each new device to be tested due to the unique footprint for each product layout; this can be a very significant cost factor when many different product layouts must be tested.
Flexible Circuits are now discussed. This test contact method utilizes a Polyimide based flexible circuit with exposed trace lines that are pressed against the DUT contact lines by applying downward pressures using an air initiated or solid tube. This temporary contact then serves as an interface between the electronic test system and the DUT. Typically the flexible circuit is held in position by being glued to a mechanical block that has the ability to be correctly positioned using extraneous hardware incorporated with the block mounting design. Each flexible circuit can contain as few as 80 contact trace lines or up to 256 lines depending on the specific application. This design is very low cost per contact line when compared to probe cards. This design also offers advantages of having the ability to replace and align an individual flex assembly very quickly when compared to probe card based test heads. This design approach also can have numerous disadvantages:
Poor Contact Performance related to surface area. Since electrical contact is being achieved by pressing together two basically flat surfaces, intermittent contact performance can be problematic. A general rule of thumb is that as contact surface area decreases, so does contact performance.
Poor Contact Performance related to deterioration or damage of contact trace lines on product. The flexible circuits used for temporary test contact can become contaminated or damaged, or in some cases be very abrasive/rough, which in turn will damage the contact trace surfaces of DUT which greatly limits the number of head down contacts that result in adequate test contact performance needed to successfully complete testing.
Poor Contact Performance related to alignment of flex assemblies. For each test head aligned there can be reduced contact performance due to positioning differences between flex assemblies. The more accurate the alignment of the entire group of flexes per test head, the greater the tool positioning error that can be tolerated and still achieve usable contact performance. A general rule of thumb is: as the pitch and line sizes are reduced, the greater the accuracy required for flex alignments. Most 100 to 120 micron pitch flex based test heads can function with tool accuracy at ±10 microns. Recent 75 micron pitch products require tool positioning accuracy less than or equal to 5 microns to achieve usable contact performance.
A final disadvantage is that Intensive Manual Alignments are required. With this current contact method it is required that manual mechanical alignments be performed for each contact trace tab group that exists on the product layout. This can range in tab group counts from as few as 16 to as many 104 individual alignments required for each product specific test head assembly. These alignments are generally repeatable after performing replacement of a test head for product swap, but at the least requires extra maintenance intervention time needed to verify these individual alignments.
What is desired is a new test method and test probe card that remedies the disadvantages found in the prior art methods and test probe cards as discussed above.